Development and research of solid-state light emitting devices in an ultraviolet range having wavelengths of lower than 380 nm would be expected to find use not only for information recording fields inclusive of DVDs but also for creation of a lot of new industries in preservation-of-the-environment fields such as sterilization as well as medical fields.
Light emitting devices for short wavelengths in the ultraviolet range are usually fabricated by making the most of wide-band gap semiconductors, but so far development and research of mixed crystals based on aluminum nitride and gallium nitride are in progress. However, aluminum nitride has a band gap of 6.0 eV to enable fabrication of light emitting devices on the wavelength order of 200 nm, yet it makes fabrication of good-quality, high-purity crystals not easy and a new approach to development of light emitting devices is still in demand.
On the other hand, wide-band gap semiconductor candidates other than aluminum nitride for solid-state light emitting devices on the emitting wavelength order of 200 nm include diamonds, cubic boron nitrides (hereinafter referred to as cBN) and hexagonal boron nitrides (hereinafter referred to as hBN) that are now under development and research for possible applications.
For hBN among others, it has now been found by the inventors that it possesses a direct transition type of semiconductor characteristics having a band gap of 5.97 eV and has high potentials for application as high-efficiency light emitting materials in the deep ultraviolet range to light emitting devices such as semiconductor lasers (for instance, see Patent Publication 1 and Non-Patent Publication 1).
The material hBN, because of being chemically stable by reason of good corrosion resistance and high melting points, is successfully used as electrically and thermally insulating material, but so far there have been no or little studies made while paying attention to the optical physical properties of hBN. This is primarily because it is difficult to obtain hBN in the form of a good single crystal: there has been no successful preparation of high-purity single crystals in the art.
That is, recent attention to the property of hBN well fit for deep ultraviolet emitting material lies in Patent Publication 1 revealing the synthesis of high-purity single crystals, and much about the optical properties of hBN.
High-purity hBN single crystals are obtained by recrystallization of the raw material boron nitride using as a solvent the nitride of a high-purity alkali metal and an alkaline-earth metal (such as barium, and lithium) as well as their boronitride, with high-brightness ultraviolet emissions at or near 215 nm. For this process the atmosphere used under the synthesis conditions and the high purity of the solvent used are important. Still, the boronitrides of the alkali metal and alkaline-earth metal used as the solvent are hard to work with because of reacting readily with water and oxygen, and require for the synthesis to be carried out in a sealed vessel under high temperature and pressure in order to prevent decomposition and oxidization of the solvent.
On the other hand, a growing solvent for bulk crystal synthesis by a solvent process takes a role of dissolving the raw material that is a solute in the solvent at high temperature, helping crystallization of the raw material upon recrystallization in a low-temperature region, and a choice of proper solvent is an imperative challenge for allowing the precipitating crystal to have higher purity or be devoid of defects, and letting the synthesis process have higher efficiency.
In view of such a challenge, the inventors have now discovered that high-purity hBN single crystals are obtainable by use of transition metal-base solvents such as nickel, more exactly a Ni—Mo alloy, without recourse to the aforesaid high-purity alkali metal (for instance, see Patent Publication 2 and Non-Patent Publication 2). With the transition metal-base solvents that are stable even under less than 1 atm, it is possible to synthesize high-purity hBN, for which a high-pressure process has been needed so far, by recrystallization from the solvent under 1 atm.
However, the above method makes it possible to obtain hBN crystals only in a very hard-to-work-with thin film form. The material hBN is in a layer compound form, and so stacking faults by mechanical deformation.
The crystal growth solvent selected must have good enough solubility with respect to the solute, and there would otherwise be no good-quality crystal obtained. In this regard, a problem with a Ni solvent is that its solubility with respect to boron that forms BN is relatively high, but its solubility with respect to nitrogen remains low. The fact that the Ni—Mo base solvent provides a good quality crystal could be due to the addition of Mo to Ni resulting in an improved solubility with respect to nitrogen (for instance, see Non-Patent Publication 3).
If there is a solvent that makes nitrogen solubility much higher than the Ni—Mo base solvent, it would work more for recrystallization of hBN.
Patent Publication 1: JP(A) 2005-145788
Patent Publication 2: JP(A) 2008-007388
Non-Patent Publication 1:
K. Watanabe, T. Taniguchi and H. Kanda, Nature Materials, 3, 404 (2004)
Non-Patent Publication 2:
Y. Kubota, T. Taniguchi, K. Watanabe, Jpn. J. Appl. Phys. 46, 311 (2007)
Non-Patent Publication 3:
C. Kowanda, M. O. Speidel, Scripta Materiallia. 48, 1073 (2003)